The present invention concerns a process for the preparation of multi-element metal oxide powders useful as precursors for the preparation of high-temperature superconducting ceramics.
Although the phenomenon of superconductivity is long known, it was just in the recent past that a major breakthrough was reached by the development of copper-containing mixed oxides which show extraordinarily high transition temperatures (T.sub.c). Although these mixed oxides usually consist of the oxides of at least three metals, the superconductors form a single homogeneous phase. In the production of such a superconducting phase, therefore, care has to be taken that the starting materials, e.g. the single metal oxides, are intimately mixed, so that upon firing a uniform product can be obtained.
Thus, for example metal oxides or carbonates used as a starting material are mixed in the desired stoichiometric ratio and heated at a temperature of about 800.degree. to 1000.degree. C. for about 20 hours. The resulting material is then reground and heated again under similar conditions.
Other processes are described in Inorganic Chemistry 1987 (26), pp. 1474-1476, and are known as the carbonate, the citrate and the oxalate route. In these cases solutions of metal salts are prepared and the metals are precipitated as carbonates, citrates or oxalates which are then dried, powdered and heated, whereby the mixed oxide phase is formed.
All these processes require quite a number of steps until a relatively homogeneous metal oxide phase is obtained. Nevertheless, it cannot be assured that even very small particles possess the same homogeneousness as the product as a whole, if such very small particles can be obtained at all.